Gamma thermometers are well known in the nuclear industry and, in general, they measure gamma radiation to determine the power output of a nuclear reactor. The gamma thermometers measure the heat generated in metal when exposed to gamma radiation and, thus, indirectly determine the power level of the nuclear device.
One type of gamma thermometer, such as the instrument shown in U.S. Pat. No. 4,298,430, uses thermocouples to measure heat produced by gamma radiation and it is constructed with an elongate metal rod disposed within a metal sheath. Both the sheath and the rod are usually constructed of stainless steel and several annular recesses are formed on the surface of the rod. When the sheath is placed over the rod, annular chambers are formed by the recesses and the sheath which are resistant to heat transfer. Thus, the chambers resist radial flow of heat out of the gamma thermometer in the region of the chambers.
The rod usually includes a central longitudinal bore which serves as a conduit for electrical wiring necessary to construct thermocouples within the rod. A separate thermocouple element is placed in the bore for each separate annular chamber with one thermocouple element leading to one annular chamber. Each thermocouple element includes at least two lead wires for forming the thermocouple and insulation between the wires. The wires and the insulation for each separate thermocouple element are separately encased in a protective metal sheath and, again, this metal sheath is usually stainless steel. Since each thermocouple element is leading to a different chamber, each element is a different length.
There are numerous variations in the construction of these thermocouple elements. For example, a thermocouple element may consist of two wires of different materials enclosed within a sheath that join to form one thermocouple or the element may include two wires of the same material with a small segment of wire of a different material at the ends of the wires inserted to form two thermocouple junctions. Some thermocouple elements may include three wires, such as the element disclosed in U.S. Pat. No. 4,313,792, for creating two or more thermocouple junctions in the vicinity of the chamber. However, in each of these examples, a separate thermocouple element extends to the vicinity of a separate chamber.
In operation, gamma thermometers detect radiation by measuring heat. The gamma radiation is absorbed by the center rod of the gamma thermometer and the rod is heated. The gamma thermometer is usually immersed within a reactor in a coolant so that the thermal energy of the rod flows radially outwardly from the rod to the coolant. This radial heat flow is blocked within the region of the chambers and this region of the rod is heated to a higher temperature than the non-insulated portions of the rod. The thermocouples are placed in the rod to measure the temperature of the rod within the chambers either as an absolute temperature or as a relative temperature, or both. This temperature information is then used to calculate the power of the radiation.
Such conventional gamma thermometers have worked well, but they are difficult and expensive to produce primarily because of the small size of the components. In many applications, to produce a useful instrument, the elements forming the thermocouples must be very small in cross-section. For example, in some extreme applications, the entire gamma thermometer must have a cross-section of less than two millimeters. Even in more typical applications, the overall cross-sectional dimension of the gamma thermometer will be about 8 mm, and the cross-sectional diameter of the sheath covering the individual thermocouple elements will be dimensioned on the order of 0.5 mm. Since the wires must be contained within the sheath, the cross-sectional diameter of the thermocouple wires within the sheath must be about 0.1 mm. The expense of the known gamma thermometer is related in part to the difficulty and expense of producing components of this small size.
The present invention reduces the expense and difficulty of manufacturing the gamma thermometer by providing a design that reduces the overall number of wires necessary to construct the gamma thermometer. A conventional gamma thermometer will have at least two thermocouple wires for each chamber region to be monitored and, thus, will have a total wire count of at least twice the number of chambers. The present invention, in one embodiment, utilizes only one wire per chamber plus one additional common wire so that the total wire count is at least equal to the number of chambers plus one. The present invention also reduces the number of thermocouple element sheaths compared to conventional gamma thermometers. In a conventional gamma thermometer a separate thermocouple element is provided for each chamber. Thus, the total number of sheaths for thermocouple elements is equal to at least the number of chamber regions monitored. In one embodiment of the present invention, an integrated thermocouple pack is provided so that only one sheath is required for all of the thermocouple wires. In a conventional gamma thermometer each thermocouple element is made separately and is then laboriously assembled into the gamma thermometer. In the present invention, the integrated thermocouple pack is made as a single unit and then the single unit is assembled into the gamma thermometer. Since the present invention requires fewer wires and sheaths than a conventional gamma thermometer, the overall size of thermocouple elements can be made to have a much smaller diameter while using the same wire sizes, or the pack can be made to have the same diameter as a conventional gamma thermometer pack, but larger wire sizes may be used. Thus, significant advantages are achieved by the present invention over conventional gamma thermometers in terms of either reduced size for the same cost or the same size for a reduced cost.
In accordance with the preferred embodiment of the present invention, a probe is provided for detecting heat. The probe includes an outer elongate sheath having a cylindrical cross-section with an elongate rod have a cylindrical cross-section fitted within said sheath. The rod is constructed of stainless steel that absorbs gamma radiation and produces heat. A plurality of annular recesses are formed on the surface of the rod and are spaced apart one from the other along the length of the rod. These recesses are disposed adjacent to the gamma thermometer sheath and form annular chambers that are resistive to heat flow. An axial bore extends axially into the rod and is positioned to extend within the cylinders defined by the annular chambers. The bore contains an integrated thermocouple pack which also extends through the cylinders defined by the annular chambers.
The integrated thermocouple pack includes a thermocouple pack sheath that is dimensioned to fit within the bore, and a common wire and a plurality of thermocouple wires are disposed within the sheath. The common wire is constructed of a first conductive material and it extends through the cylinders defined by the annular chambers. The thermocouple wires are also constructed of the first conductive material, but at least a portion of the thermocouple wires is constructed of a second conductive material so that a thermocouple is formed by the junction of said first and second materials. In the preferred embodiment there are a plurality of thermocouple wires at least equal in number to the number of annular chambes to be monitored and the thermocouple wires are disposed in the pack sheath to extend through the cylinders defined by the annular chambers and are connected to the outer end of the common wire. The thermocouple wires are also constructed and dimensioned to form at least one thermocouple couple proximate to each annular chamber with one of said thermocouple wires forming a thermocouple proximate to one of said chambers for producing electromotive forces corresponding to the temperatures of the rods proximate to said annular chambers. In the preferred embodiment, each thermocouple wire is constructed to form two thermocouple junctions, one inside a cylinder defined by an annular chamber and the other being disposed outside the cylinder. Electrical insulation is disposed within the pack sheath for insulating the first and second type wires, one from the other, and from the pack sheath itself. A heater cable can also be inserted within the integrated pack to provide in situ calibration and reactor coolant level detection.
Although the probe has been described herein with reference to gamma thermometers, it will be understood that the probe has other applications. For example, U.S. Pat. No. 4,406,011 discloses that gamma thermometers may be used as coolant level detectors in nuclear reactors.